The invention relates to a SQUID measurement apparatus for detecting weak magnetic field signals that change over time such as from a biomagnetic field source. The apparatus includes at least one measurement channel having a SQUID, a superconductive flux transformer arranged in front of the SQUID that has at least one detection loop for receiving the field signals and at least one connection conductor connected to this loop. Shielding means are assigned to the connection conductor of the flux transformer. Such a SQUID measurement device is disclosed in EP-B-0 185 186, for example.
Using superconductive quantum interferometers, which are also known as "SQUIDs" (which is an abbreviation of Superconducting QUantum Interference Devices), extremely weak magnetic fields can be measured (cf. "IEEE Trans. El. Dev.," Vol. ED-27, No. 10, Oct. 1980, pages 1896 to 1908, for example). For this reason, medical diagnostics is considered a preferred area of application for SQUIDs, since the biomagnetic signals that are produced, e.g. the magnetic fields generated by the human heart or the human brain (magnetocardiography or magnetoencephalography), provide field intensities that are only in the pT range.
An apparatus for detecting and processing such weak magnetic fields that are dependent on time and special location contains at least one measurement or detection channel. This channel has a so-called flux transformer with at least one antenna formed as a gradiometer or magnetometer and, if necessary, it also has a coupling coil. Furthermore, a SQUID circuit is arranged after the flux transformer, which generally includes a SQUID, a modulation coil integrated into the circuit, and an amplifier and evaluation electronics. Except for the amplifier and the evaluation electronics, the above-mentioned components are formed from a superconductive material and are housed in a corresponding cryosystem in order to provide a superconductive operating environment. The antenna is formed from at least one detection loop for detecting the field signals of the field source. The corresponding measurement signal reaches the SQUID circuit via at least one superconductive connection conductor connected with the detection loop. For measuring the flux coupled into this circuit, or for measuring flux gradients, both RF SQUIDs (high-frequency or radio-frequency SQUIDs) as well as DC SQUIDs (direct current SQUIDs) are used. A measurement device having a plurality of correspondingly structured measurement channels is disclosed, for example, in the publication "Cryogenics," Vol. 29, Aug. 1989, pages 809 to 813.
In the operation of open SQUID measurement devices, for making biomagnetic measurements, for example, in which the field sources are located outside the cryosystem there are significant problems which result from high-frequency electromagnetic interference that radiates into the field-sensitive parts of the measurement device. The high-frequency interference, which mainly reaches the SQUID via connectors of the flux transformer which are difficult to filter, blur the structure of the non-linear SQUID characteristic and thereby reduce the signal delivered by the SQUID. This results in increased noise in the measurement device, in an undesirable detection of frequency and amplitude of the most unstable interference generator, or in some cases even in complete failure of the corresponding measurement channel.
Since no effective high-frequency filters can be inserted in the lines of the flux transformer, and in addition, since most measurement structures cannot be designed to properly take high frequencies into account due to the low frequencies to be detected, passive shielding techniques have been mainly used until now. However, these shielding techniques were generally not carried out in the immediate vicinity of the flux transformer, particularly in the vicinity of the detection loop, since at this location, the use of normal conductive materials leads to an increase in noise in the measurement channel due to thermal fluctuations, while superconductive materials distort or completely displace the field signal to be detected. For this reason, the measurement device disclosed in EP-B-0 185 186, mentioned above, provides shielding only by means of a superconductive track along the connection conductor of the flux transformer.
Because of the above-mentioned problems, the entire measurement space may be housed, for example, in a shielding chamber (cf. the reference from "Cryogenics" as cited, or "Biomagnetism--Proceedings Third International Workshop on Biomagnetism, Berlin, May 1980," pages 51 to 78). In this case, however, complicated filters for electrical supply lines or optical data transmission parts are required. Also, another known method includes temporary shielding with aluminum foils on the outside walls of a SQUID measurement system. Such shielding is not very effective, however, and furthermore, increases the noise.
The problem with the prior art is that there is no measurement device having detection loops forming magnetometers or gradiometers that provide simplified shielding measures in comparison to known methods of shielding.